Tagging buried irrigation lines for future identification?

HI,
I have an irrigation question I hope someone can help me with. I am totally rebuilding the 17 year old irrigation system, correcting the mistakes made by the 'who-knows-who' that installed the system originally. I have 3 separate 1/2" (Toro blue stripe) irrigation lines that are in a common trench, from the valve box to three separate areas. The three lines are in a common trench until they branch out to their designated planting areas. Then two lines in a different trench to another area. I have one line for native desert plants, another for native trees, and still another for a raised bed vegetable gardens. Due to the irrigation requirements of each type of plant, I need three separate circuits, valves, timer settings etc. I am using Netafim dripline on the trees & veggies, and point emitters on the shrubs.
I have taken reference photographs of the trenching with the lines laid in, so I will know where the lines are buried, but need a positive way to ID the individual lines years down the road.
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Is there any way I can mark the 1/2" tubing so I will know which one is which, should I need to access them sometime in the future? I have thought of spray painting each 1/2" line in a different bright color, but am not sure it would last in the alkaline ground, or if the paint would cause the tubing to degrade over time. I have also considered a loose wrap of the lines in different color flagging tape.
Suggestions?
Thanks in advance